The present invention pertains to optical delay lines. In particular, one embodiment of the present invention relates to a grating-based, phase control optical delay line used, for example and without limitation, in Optical Coherence Tomography (xe2x80x9cOCTxe2x80x9d) and Optical Coherence Domain Reflectometry (xe2x80x9cOCDRxe2x80x9d).
A low coherence, optical interferometer has been used in various apparatuses to study scattering media. FIG. 4 shows a block diagram of an Optical Coherence Tomography (xe2x80x9cOCTxe2x80x9d) system. As shown in FIG. 4, OCT system 50 includes an interferometer with reference path 5, sample path 10, splitter/combiner 15 (one embodiment of splitter/combiner 15 comprises a 50/50 beamsplitter, or a 3 dB coupler if the interferometer is embodied using optical fibers), low coherence radiation source 20, detector 25, and processor 30. Scanning optical delay line 35 (ODL 35) is located at an end of reference path 5 of the interferometer. As further shown in FIG. 4, sample path 10 includes probe module 40 to direct radiation to sample 45, and to collect radiation scattered from sample 45. As is still further shown in FIG. 4, detector 25 combines a sample beam reflected from sample 45, and a reference beam reflected by scanning ODL line 35. Then, whenever an optical path length mismatch between sample path 10 and reference path 5 is less than a coherence length of low coherence radiation source 20, interference between the sample beam and the reference beam occurs. As is well known in the art, if the optical path length of the reference beam is known when detector 25 senses the interference signal, the optical path length of the sample beam can be measured within the accuracy of the coherence length of the low coherence radiation source.
Several designs of an optical delay line for use in the reference path have been disclosed in the art. As described in an article entitled xe2x80x9cOptical Coherence Tomographyxe2x80x9d by Huang et al. in Science, Vol. 254, 1991, pp. 1178-1181, a mirror is used to reflect the reference beam back to the detector. In accordance with this article, depth information from the sample medium is acquired by varying the optical path length of the reference path by moving the mirror using a stepper motor. U.S. Pat. No. 5,321,501 (Swanson et al.) discloses a change to the design of Huang et al. in which the mirror is replaced by a retroreflector to improve optical alignment stability, and the stepper motor is replaced by a galvanometer to increase the scan speed to a degree where tomographical images of living tissue became feasible.
U.S. Pat. No. 6,111,645 (Tearney et al.) discloses a change to the design of Swanson et al. in which the moving retroreflector is replaced by a grating-based, phase control, optical delay line. U.S. Pat. No. 6,111,645 is incorporated by reference herein. This design change further increases the scanning speed over that disclosed in Swanson et al., and also enables independent control of the phase and group delay of produced by the reference path. However, the grating-based, phase control, optical delay line disclosed by Tearney et al. has been limited thusfar to use in a laboratory environment.
In light of the above, there is a need for an optical delay line that can provide high scanning rates, and that is suitable for use in optical interferometers to provide tomographic images of living tissue.
Embodiments of the present invention advantageously satisfy the above-identified need in the art. In particular, one embodiment of the present invention is an optical delay line (xe2x80x9cODLxe2x80x9d) that is suitable for use in optical interferometers to provide tomographic images of living tissue. Specifically, a first embodiment of the present invention is an optical delay line that comprises a plurality of optical elements in optical communication with each other, wherein: (a) at least one of the plurality of optical elements is capable of spatially dispersing a spectrum of an optical signal to provide a spatially dispersed optical signal; (b) at least one of the plurality of optical elements is adjustable to affect one or more of a phase delay and a group delay of an optical signal; and (c) at least one of the plurality of optical elements compensates for polarization introduced into the optical signal by others of the optical elements. In addition, a second embodiment of the present invention is an optical delay line that comprises: (a) a collimator lens system; (b) a grating disposed to receive radiation output from the collimator lens system; (c) a collector lens system disposed to receive at least a portion of radiation diffracted by the grating; (d) a rotatable mirror disposed substantially at a back focal plane of the collector lens system; and (e) a reflector disposed to reflect at least a portion of radiation diffracted by the grating; wherein the collimator lens system, the reflector, and an output end of an optical fiber are affixed in a unit, which unit is movable by a translation mechanism.
Another embodiment of the present invention is an optical interferometric imaging system to be used, for example and without limitation, in a clinical setting. In particular, one embodiment of the present invention is an optical interferometric imaging system for imaging a sample that comprises: (a) an optical source capable of producing an optical signal having an optical spectrum; (b) an interferometer in communication with the optical source; (c) a detector in optical communication with the interferometer; and (d) an optical delay line in optical communication with the interferometer that comprises a plurality of optical elements in optical communication with each other, wherein: (i) at least one of the plurality of optical elements is capable of spatially dispersing a spectrum of the optical signal to provide a spatially dispersed optical signal, (ii) at least one of the plurality of optical elements is adjustable to affect one or more of a phase delay and a group delay of the optical signal, and (iii) at least one of the plurality of optical elements compensates for polarization introduced into the optical signal by others of the optical elements.